Dark-field contrasting methods are being increasingly employed for the microscopic inspection of semiconductor substrates, since such methods permit better recognition and measurement, for instance, of the line widths of the microstructures on the substrates than is possible with bright-field observation. For this purpose, conventiona microscopes are provided with special objectives around which is placed a so-called dark-field condenser in the form of an annular mirror. This annular mirror focuses onto the object an illuminating light beam which is conducted concentrically around the objective and illuminates the entire image field.
It is also known in the art to use what are called light scan microscopes in order to inspect semiconductor substrates. These instruments, known in part also as laser scan microscopes (LSM), produce a light spot which is generally diffraction-limited and which is moved line-by-line over the object by means of deflectin mirrors. In these microscopes, the objective serves to focus the scanning light beam on the object.
In order to produce a dark-field image with such microscopes, there is used the signal of a detector which is arranged alongside the objective and therefore outside the aperture of the illuminating optical system. Such a light scan microscope is disclosed, for instance, in U.S. Pat. No. 4,441,124 of Heebner et al., granted Apr. 3, 1984, in which the entire light detected by the dark-field condenser (annular mirror) is fed to a single detector, which supplies the dark-field signal. The dark-field images thus obtained are, however, not optimal with respect to ease of recognition of the line structure extending in two pronounced preferred directions on the semiconductor substrate. In particular, it is not readily possible to bring out defects or particles of dirt on the surface of the substrate clearly against the structures applied.
U.S. Pat. No. 4,460,273 of Koizumi et al., granted July 17, 1984, shows a light scan microscope containing two dark-field detectors arranged alongside the objective, the line between them extending perpendicular to the line structures on the substrate surface, as illustrated in FIGS. 16 and 17 of this patent. With such an arrangement, it is possible to suppress effecively only structures having a single preferred direction. Thus unambiguous discrimination between structure and defect is not possible. Furthermore, the detector arrangement must always be directed at right angles to the structures on the substrate, making the operation or use of the instrument difficult.
European Pat. No. 0,011,709 of Wagner, published June 23, 1982, describes a microscope for inspection of semiconductors, the objective of which contains a plurality of light guides arranged in the rear focal plane of the objective itself. These light guides can be combined in sector form, and conduct light to one or more detectors.
The effective aperture of the objective is limited by such an arrangement in the manner that the diffraction-limited focusing of the illuminating light conducted over the objective which is required in light scan microscopes is no longer assured, i.e., the resolving power is impaired. Furthermore, it is difficult to integrate a larger bundle of light-guide fibers into the objective without detriment to the ease of handling, as in interchange, or application to a turret. That European patent does not specifically indicate the manner in which the signals of several detectors are to be processed in order, for instance, to emphasize defects or particles of dirt in the dark-field image.
The object of the present invention is to provide a method of inspecting incident-light objects such, for example, as wafers in the semiconductor industry, which assures the best possible recognition of defects, impurities, etc., and to provide apparatus suitable for this purpose which may be constructed without great expense by a substantial use of standardized components already available.
This object is achieved in the manner set forth in the claims.
The invention makes use of the fact that particles of dirt such as dust, defects, and, for instance, torn edges on semiconductor structures disperse light without any pronounced preferred direction, while the scattering characteristic curve of well-defined smooth edges and surfaces has a pronounced preferred direction. Therefore, if at least three and preferably four or more detectors are arranged in a circle around the objective (or around the condenser, in the case of substage illumination), and if these condensers simultaneously supply signals of comparable size, this constitutes an indication of scattering caused by particles of dust or defects. The signal of all detectors resulting from a logical "and" operation therefore supplies an image of the object in which all structures having smooth edges are suppressed, regardless of the orientation of the edges. Automatic inspection of masks and wafers for dust and defects is thus substantially facilitated.
When four detectors are used, the signals of opposite detectors can furthermore be subtracted from each other and the sum of the two different signals can be used to display the image. It is thus possible to produce another dark-field image containing different information with the same detectors used to recognize impurities. In this other image, object structures such as lines and conductive paths are displayed three-dimensionally (pseudo-3D) and thus it is possible to make a decision as to whether a structure of an object is elevated or depressed (recessed).
Light scan microscopes usually also have, in any event, a bright-field detector, for instance in the form of a multiplier. It is advisable to superimpose the signal from this detector on the signal produced by the logical "and" operation in the manner of a false-color display. Particles of dust and defects then appeaar clearly emphasized in the bright-field image of the specimen, so that a simple association in space between the structure and the defects is possible.
In the apparatus of the present invention, a traditional dark-field objective is used for reflected-light displays. In the annular illumination channel surrounding the lens, which is not necessary for illumination purposes when used as a light scan microscope, three or more detectors are installed.
For the display of transparent objects such as, e.g., masks in transmitted light, the detectors are arranged in a suitable manner around the condenser of the microscope optical system, or are fastened to it.
An optical system which collects light separately for each detector is preferably arranged in front of the detectors. This makes it possible to keep the detector surface and thus the capacitance of the detector diodes small, so that the highest possible scanning frequency up to well into the megacycle region can be obtained with minimum detector noise.
Also it is advantageous for the electronic circuit to contain a comparator for the logical operation of the detector signals in each signal channel, i.e., in the signal path in front of the logic circuit. In such case it is possible, by suitable adjustment of the comparator threshold, so suppress dust or defects below a certain size in the image, if this is desired.
Further advantageous embodiments of the invention will be apparent from the following description read in conjunction with the drawings.